This invention relates to novel complexes of the macrocyclic polyether, 1,4,7,10,13,16-hexaoxacyclooctadecane, and to a process for the isolation of 1,4,7,10,13,16-hexaoxacyclooctadecane from mixtures containing this polyether in which the novel complexes are advantageously employed to effect separation of the polyether from the mixture.
Macrocyclic polyethers such as 1,4,7,10,13,16-hexaoxacyclooctadecane have demonstrated ability to solubilize or complex metal cations in both polar and relatively non-polar media, which, in turn, makes them valuable reagents in a variety of synthesis and separation processes. In this regard, 1,4,7,10,13,16-hexaoxacyclooctadecane, which is also known by its trivial name "18-crown-6,"has special utility in the complexing of lower alkali metal cations e.g. potassium, in such diverse media. The disclosure of metal cation complexing properties for 18-crown-6 has stimulated considerable technical interest and, as a result, a number of synthetic techniques have been proposed for preparing the macrocyclic polyether. These preparative processes for 18-crown-6 include:
(1) Catalytic oligomerization of ethylene oxide, see U.S. Pat. No. 3,928,386. PA1 (2) Reaction of tetraethylene glycol with bis(2-chloroethyl) ether in the presence of potassium hydroxide and tetrahydrofuran without addition of water, as described in "Synthesis" 1976, 515-516. PA1 (a) contacting the mixture with dimethylcarbonate or dimethyloxalate thereby causing the 18-crown-6 to react with the dimethylcarbonate or dimethyloxalate and form, respectively, an 18-crown-6-dimethylcarbonate or an 18-crown-6-dimethyloxalate complex substantially in the form of a dispersed solid in the resulting solution of non-complexed material; PA1 (b) separating the dispersed complex from the resulting solution; and PA1 (c) disassociating the separated complex to afford uncomplexed 18-crown-6.
While the aforementioned preparative techniques for 18-crown-6 show a considerable variation in product yield and cost of starting materials, all share the same disadvantage from a practical or commercial standpoint in that difficult and/or commercially unattractive procedures are required to isolate the 18-crown-6 from the reaction mixture.
The technique disclosed for isolating 18-crown-6 from the reaction mixture obtained by process (1) above involves chromatographic separation on acid-washed alumina or silica and elution with readily volatile hydrocarbons. This isolation procedure is not particularly attractive from a commercial standpoint because the adsorption capacities of alumina or silica for the 18-crown-6 are rather low and the used alumina or silica must be regenerated or discarded and replaced by fresh alumina or silica.
In the reference process (2) above, potassium chloride and tetrahydrofuran are removed from the reaction mixture and the resulting product is distilled to afford a crude 18-crown-6 overhead product. Subsequently the distilled 18-crown-6 is mixed with acetonitrile and the mixture obtained is cooled to a very low temperature, e.g., -45.degree. C., to precipitate the 18-crown-6-acetonitrile complex which forms on addition of the acetonitrile. The precipitated complex is then filtered off and the acetonitrile is evaporated from the filtered complex at sub-atmospheric pressure with gentle heating. The residue is then distilled to obtain the 18-crown-6 as a distillate. One disadvantage of using acetonitrile in accordance with reference process (2) for isolation of the 18-crown-6 is high solubility of 18-crown-6-acetonitrile complex in acetonitrile at ambient temperatures. Because of this high solubility, very low temperatures must be used to precipitate the complex from the excess acetonitrile and even then the complexed 18-crown-6 is obtained in rather low yield. A further disadvantage of reference process (2 ) above is that it includes at least one step wherein the 18-crown-6 is distilled overhead, and therefore, additional measures must be taken to avoid the occurrence of powerful and destructive explosions which are known to occur during the distillation of 18-crown-6, see "Chemical and Engineering News," Sept. 6, 1976, page 5 and Dec. 13, 1976, page 5.
The isolation of 18-crown-6 from reaction mixtures can also be carried out using nitromethane as the complexing agent, as described and claimed in co-pending application, Ser. No. 38,039, filed May 11, 1979, Common Assignee. The isolation can be carried out at ambient or somewhat lower temperature (because of the lower solubility of the 18-crown-6-nitromethane complex in nitromethane). The 18-crown-6 need not to be distilled in order to separate it from the 18-crown-6-nitromethane complex as nitromethane can be removed from the complex by heating the complex at sub-atmospheric pressure. However, the use of nitromethane implies some drawbacks, expecially in large scale operations, due to its toxicity and explosion danger when subjected to higher temperatures. Furthermore, the complex contains two molecules of nitromethane per molecule of 18-crown-6, so that two moles of the complexing agent must be removed from each mole of the separated 18-crown-6-nitromethane complex obtained.
From the foregoing, it is apparent that considerable advantage would be obtained if a simple and cost effective means could be found for isolating 18-crown-6 from reaction mixtures which avoids the commercial impracticalities and potential hazards associated with previous separation techniques.
Various complexes between 18-crown-6 and other compounds have been prepared and are disclosed, for example, in U.S. Pat. Nos. 3,562,295, 3,687,978 and 3,997,562 and in Chem. Abst., Vol. 82, No. 124273d. Complexes of 18-crown-6 and dimethyl acetylenedicarboxylate are disclosed in:
Goldberg, Acta Cryst., Sect. B, 31, 754 (1975)
Timko et al., JACS, 99, 4207 (1977)
British Patent Specification 1,481,671 Neither the use of such complexes to isolate 18-crown-6 from reaction mixtures or the presence of complexes of 18-crown-6 and dimethylcarbonate or dimethyloxalate are disclosed by these references.